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Silicon wafer bonding for MEMS manufacturing


08/01/1999







Various silicon wafer-bonding methods are crucially important to the success of MEMS manufacturing. Bond-specific equipment has evolved along with these methods.

The manufacture of all leading-edge microelectromechanical systems (MEMS) requires wafer-level bonding of a silicon wafer to another silicon substrate or a glass wafer. This provides a first-level, wafer-packaging solution that makes these products economically viable.

Consider that aligned silicon wafer bonding is an enabling technology for Motorola's automotive airbag accelerometer; the company has shipped more than 10 million units. In this device, a silicon top cap is bonded at the wafer level to the triple-level, polysilicon, surface-micromachined accelerometer using a low-temperature glass frit [1]. This process provides mechanical protection of the sensor and allows use of conventional leadframe assembly and injection-molded plastic packaging. At the same time, the silicon capping process provides the necessary damping and shock protection needed for the accelerometer, since the capping may be performed at a controlled pressure and temperature. Without this innovative use of silicon wafer-bonding technology, it is unlikely that this MEMS product would have achieved such market success.

Silicon wafer-bonding physics

Several wafer-bonding techniques achieve different bonds - anodic, direct, or intermediate-layer bonds [2]. The latter includes eutectic and glass-frit bonds. Typical process conditions used for some of these bonds are shown in the table.

The general process of wafer bonding is a three-step sequence consisting of surface preparation, contacting, and annealing.

Surface preparation is important because bond quality depends strongly on surface conditions; precautions must be taken to ensure that surface contamination or particles do not preclude a good bond. A 1µm particle, for instance, can cause a void as large as 1cm in diameter when direct-bonding 200mm wafers [3]. In addition, it is necessary to ensure that surface roughness is within 5Å, and that sample flatness is ~5µm, the latter determined on a 100mm wafer [4].

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With appropriately clean surfaces, materials are bonded via contact; close contact is ensured by pressing them together. For silicon-direct wafer bonding, a high-temperature step is subsequently performed to strengthen the bond.

Anodic bonding

Anodic bonding (Fig. 2) relies on electric charge migration to bond wafers. This usually involves a silicon wafer and a glass wafer with a high content of alka* metals [1]. For instance, Pyrex borosilicate glass, a typical material for this application, has a Na2O content of 3.5%. With anodic bonding, the presence of mobile metals is exploited by applying a high negative potential to the glass to attract Na+ ions to the negative electrode where they are neutralized [5]. Such removal permits the formation of a space charge at the glass-silicon interface; this creates a strong electrostatic attraction that holds both pieces firmly in place. The bond is performed at up to 500°C, increasing the mobility of the positive ions. Furthermore, driven by the existing electric field, oxygen from the glass is transported to the glass-silicon interface, where it combines with silicon to form SiO2, which creates a permanent bond.


Figure 2. A negative potential applied to a borosilicate glass heated to 500°C allows the migration of Na+ away from the wafer interface, creating a strong electric field between the samples.
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This technique, originally proposed in 1969 [6], produces uniform bonds; however, the presence of charge carriers means the bond is not compatible with active devices. The method is used for pressure sensors, solar cells, piezoresistive applications, and packaging.

A novel application involving anodic bonding was reported in 1997 [7]. The application involved fabrication of silicon micronozzles capped on both sides by Pyrex wafers to test free-jet expansion conditions. The strength of the bond and visualization of streams and other phenomena were fundamental requirements for the success of the project. A subsequent project at MIT fabricated, bonded, and tested microrocket structures that could withstand chamber pressures >1000psi.

Silicon-direct bonding

Silicon-direct wafer bonding joins two hydrophobic or hydrophilic silicon surfaces brought into contact and annealed at high temperature [4]:

  • The hydrophobic method uses a HF dip before contacting. While this preliminary step makes surface contacting more difficult compared to hydrophilic surfaces, the bond could ultimately be better.

    • The hydrophilic method uses a standard RCA clean prior to bonding. The presence of hydroxyl radicals on the mirror-polished silicon surface permits a good initial bond upon contacting. Pressing the middle of one of the wafers creates an initial point of contact that originates the bond (Fig. 3a), while mechanical spacers maintain physical separation. Removing the spacers (Fig. 3b) allows a single bonding wave to propagate from the center of the wafers with a speed that is influenced by the viscosity and pressure of the ambient gas, as well as the contact energy of the samples. The mechanical spacers are important in establishing a single bond front that propagates outward because multiple bonding waves promote warpage [8], and gases can be trapped in pockets formed by multiple waves, leading to areas of poor bonding.

    Subsequent heating dehydrates the surface and causes a number of processes to take place: Hydroxyl groups form water molecules that promote oxidation of the bonding surfaces, creating a Si-O-Si bond as the hydrogen diffuses away. At higher temperatures, oxygen also diffuses into the crystal lattice to create a bond interface that is not distinguishable from the rest of the silicon crystalline structure. In fact, it has been reported that at annealing temperatures >1000°C the strength of the bond approaches that of silicon.

    Although the high annealing temperature involved is a drawback for some applications, this technique permits the formation of cavities, as well as all-silicon, stress-free bonded structures. It is being used in fabrication of Power MEMS pioneered at MIT. For example, a micro gas turbine engine, part of MIT's turbomachinery program, is a stack of five silicon wafers - a free-standing turbine is fabricated on the middle wafer (Fig. 4) and encapsulated by two wafers on top and bottom. The whole stack includes a complex array of channels and perforations to levitate the turbine, for fuel intake for combustion and exhaust, as well as for metrology operations.

    Intermediate-layer bonds


    Figure 3. a) During direct wafer bonding, pressure is applied at the middle of one wafer, creating a single point of contact between the pieces involved. b) After removing the mechanical spacers, a bonding wave propagates toward the periphery.
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    Eutectic- and glass-frit-bonding techniques involve deposition of metallic or glass intermediate films prior to bonding. For eutectic bonding, examination of a two-component phase diagram, when there is little or no solid solubility between the components, reveals the eutectic point at the lowest melting temperature [9].

    The alloy forms by solid-liquid interdiffusion at the interface, followed by solidification upon cooling. For gold and silicon, this point is 363°C and corresponds to a eutectic composition of 2.85% Si and 97.1% Au by weight. To accomplish a good bond, silicon surface preparation must remove oxide films that can hamper diffusion of gold into silicon. In addition, once gold for the eutectic bond has been deposited, the gold must be exposed to UV light immediately before bonding to remove organic contaminants that preclude good surface contact. Similar to anodic bonding, the method uses pressure applied with the wafers at the appropriate temperature.

    The low temperature required to reach the eutectic point makes this technique attractive. However, its compatibility with ICs is a concern.


    Figure 4. MIT's micromachined turbine. (Source: C-C. Lin, M. Schmidt)
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    Glass-frit bonding creates hermetic seals using low temperatures [1, 5]. First, the process deposits and pre-glazes a thin glass layer. Then the wafers contact under pressure at the rated melting temperature of the glass, which is always <600°C. The glass is usually a lead borate with significant lead oxide content, but other compositions are more compatible with integrated active devices, making this approach very attractive.

    Intermediate-layer bonds require inspection, usually with infrared imaging, x-ray topography, or ultrasonic methods. Although infrared transmission has some resolution limits due to the applied wavelength, its simplicity of operation is remarkable, only requiring an infrared source and a charge-coupled device camera.

    Bond strength. Bond strength is critical in many applications. It can be evaluated with several techniques. For example, various methods apply loads to push, pull, shear, or peel bonds. One approach presented based on surface energy measurements usually yields consistent results [10]. This method inserts a blade of known thickness between the bonded pieces to promote the formation of a crack whose length is subsequently measured. The surface energy is then calculated by

    Y = (3/8)(Et3y2)/(L4)

    where, t = sample thickness, E = modulus of elasticity, 2y = blade thickness, L = crack length, and Y = specific surface energy.

    The only drawback to this approach is the fourth-power dependence on the length of the crack; thus, errors in the measurement of the induced crack length can translate into unacceptable surface energy values.

    Aligned wafer-bonding systems. Aligned silicon wafer bonding is a two-step process:

    • Wafers are aligned to each other in a bond aligner. One such system, the Electronic Visions EV640, can perform both wafer-to-wafer alignments for wafer-bonding applications, and mask alignments and single- and double-sided exposures for lithography applications. Once the wafers are aligned, with a typical alignment accuracy of ±1µm, they are then clamped together with an appropriate separation gap between the wafers, in a bond fixture.

      • The bond fixture is loaded into a vacuum bond chamber where the wafers are contacted together under computer control. (Separation of alignment and bonding processes for better productivity was pioneered by Electronic Visions in 1990.) In this process step (with, for example, the Electronic Visions EV560 production cluster tool wafer bonder), wafer temperatures up to 550°C may be obtained using simultaneous topside and bottomside heating of the wafer stack. This ability is particularly important in applications where precise temperature control is mandated, such as with glass-frit bonding and anodic bonding of thick glass wafers (glass being an excellent insulator). In anodic bonding applications, the bonding system may apply voltages up to 2kV, or for thermocompression bonding, may use forces up to 8000lbf (35kN).

      Advanced wafer bonding

      A number of advanced MEMS devices require the bonding of multiple silicon micromachined wafers. For instance, three wafers, all previously aligned to each other, may be simultaneously bonded to each other in a vacuum bonder chamber using silicon-direct wafer or anodic bonding (Fig. 5).


      Figure 5. Triple-stack anodic bonding of three wafers for a commercial accelerometer device from VTI Hamlin (Breed Technologies). This bonding technology was first introduced by Electronic Visions in 1992.
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      Advanced MEMS applications have also driven the development of integrated processing capability. For example, the Electronic Visions EV850 Bond System uses this process:

      • cleans two silicon wafers using a dilute ammonium hydroxide plus hydrogen peroxide chemistry with megasonic nozzle or brush clean;

        • aligns the wafers to each other using optical or wafer flat alignment;

          • brings the wafers into contact in either a vacuum or controlled ambient;

            • thermally anneals the wafers to increase bond strength; and

              • finally, inspects the joined wafers for voids using infrared imaging.

                This integrated bonding system is being used for the production of SOI-bonded wafers and silicon pressure sensor device fabrication.

                Conclusion

                Silicon wafer bonding is now established as the key enabling technology for a wide number of commercially successful MEMS products. Significant advances have been made in automated process equipment for production silicon wafer bonding, as well as the integration of key process steps into one bond system. Advanced MEMS devices require the bonding of multiple wafer stacks that can be accommodated in production wafer bonders through appropriate bond tool design.

                References

                1. S.A. Audet, K.M. Edenfeld, "Integrated Sensor Wafer-Level Packaging," Proceedings of the 1997 International Conference on Solid-State Sensors and Actuators, Chicago, Transducers '97, 1D4.09P, pp. 287-289.
                2. M.A. Schmidt, "Wafer-to-Wafer Bonding for Microstructure Formation," Proceedings of the IEEE, Vol. 86, No. 8, pp. 1575-1585, August 1998.
                3. L. Ristic, Sensor Technology and Devices, Artech House Publishers, 1994.
                4. M.A. Schmidt, "Silicon Wafer Bonding for Micromechanical Devices," Solid State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, pp. 127-131, June 1994.
                5. W.H. Ko, J.T. Suminto, G.J. Yeh, "Bonding Techniques for Microsensors," in Micromachining and Micropackaging of Transducers, eds., C.D. Fung, P.W. Cheung, W.H. Ko, D.G. Fleming, Elsevier Science Publishers, 1985.
                6. G.D. Wallis, D.I. Pomerantz, "Field-Assisted Glass-Metal Sealing," J. Appl. Physics, Vol. 40, pp. 3946-3949, 1969.
                7. R. Bayt, A.A. Ay?n, K. Breuer, "A Performance Evaluation of MEMS-Based Micronozzles," 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Seattle, WA, July 1997.
                8. D.L. Hughes, "Silicon-Silicon Direct Wafer Bonding," Second International Symposium on Semiconductor Wafer Bonding: Science, Technology and Applications, Hawaii, PV20-10, pp. 17-31, 1992.
                9. R.F. Wolffenbutel, K.D. Wise, "Low-Temperature Silicon Wafer-to-Wafer Bonding using Gold at Eutectic Temperature," Sensors and Actuators A, Vol. 43, pp. 223-229, 1994.
                10. W.P. Maszara et al, "Bonding of Silicon Wafers for Silicon-on-Insulator," J. Appl. Phys., Vol. 64, No. 10, p. 4943, Nov. 1988.

                  Authors

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                  A.R. Mirza has designed and developed silicon micromachined sensors for more than 12 years, and has held engineering management positions at Motorola and Honeywell. He is technology manager at Electronic Visions Inc., 3701 E. University Dr., Suite 300, Phoenix, AZ 85034; ph 602/437-9492, fax 602/437-9435, e-mail [email protected].

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                  A.A. Ayón works in the development and microfabrication of turbomachinery and power generation devices. He is a research scientist at the Massachusetts Institute of Technology, 77 Massachusetts Ave., Rm. 31-145, Cambridge, MA 02139; ph 617/253-6049, fax 617/258-6093, e-mail [email protected].